1. Field of Invention
The invention generally relates to a technical pen, and, in particular, one for use in a drafting instrument such as a plotter, comprising a writing tube fixed in the front end of a pen tip, into which a cleaning wire, fastened to the front end of a drop weight body, extends from the rear. The drop weight body is disposed so that it is axially movable back and forth in a limited way in an inner bore which is connected with a writing fluid reservoir, An indicator device for monitoring the writing fluid level is disposed in a buffer chamber between the front end of the inner bore and the back end of the writing tube. The indicator device activates a conveying means for transporting writing fluid from the inner bore into the buffer chamber when the writing fluid has fallen below a pre-determined level.
2. Brief description of the prior art
One writing instrument, in the form of a fountain pen, having a pump conveying means to transport writing fluid into a buffer chamber near the nib and a capacitive sensor device to monitor writing fluid level in that buffer chamber is described in German published Patent Application, DE-OS 33 21 301 which is equivalent to HERRNRING (U.S. Pat. Nos. 4,634,305 and 4,708,506). The capacitive sensor of this writing instrument is not shown in any detail, but apparently forms the front boundary wall of the buffer chamber, through which a bore extends as a connection with the nib. If such a writing instrument is held exactly vertical, the sensor will be completely wetted if writing fluid is present in the buffer chamber. However, the sensor will only partially be wetted if the writing instrument is held at an angle. Hence, there is a possibility that the sensor falsely will signal a lack of writing fluid in the buffer chamber when the writing instrument is held at an angle, whereby the conveying means will move writing fluid into the buffer chamber, even though not necessary.